1 /*
2 * Copyright (C) 2011, Google Inc.
3 * and other copyright owners as documented in the project's IP log.
4 *
5 * This program and the accompanying materials are made available
6 * under the terms of the Eclipse Distribution License v1.0 which
7 * accompanies this distribution, is reproduced below, and is
8 * available at http://www.eclipse.org/org/documents/edl-v10.php
9 *
10 * All rights reserved.
11 *
12 * Redistribution and use in source and binary forms, with or
13 * without modification, are permitted provided that the following
14 * conditions are met:
15 *
16 * - Redistributions of source code must retain the above copyright
17 * notice, this list of conditions and the following disclaimer.
18 *
19 * - Redistributions in binary form must reproduce the above
20 * copyright notice, this list of conditions and the following
21 * disclaimer in the documentation and/or other materials provided
22 * with the distribution.
23 *
24 * - Neither the name of the Eclipse Foundation, Inc. nor the
25 * names of its contributors may be used to endorse or promote
26 * products derived from this software without specific prior
27 * written permission.
28 *
29 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
30 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
31 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
32 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
33 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
34 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
35 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
36 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
37 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
38 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
39 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
40 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
41 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
42 */
43
44 package org.eclipse.jgit.internal.storage.dfs;
45
46 import java.io.IOException;
47 import java.nio.channels.ReadableByteChannel;
48
49 /**
50 * Readable random access byte channel from a file.
51 */
52 public interface ReadableChannel extends ReadableByteChannel {
53 /**
54 * Get the current position of the channel.
55 *
56 * @return r current offset.
57 * @throws java.io.IOException
58 * the channel's current position cannot be obtained.
59 */
60 public long position() throws IOException;
61
62 /**
63 * Seek the current position of the channel to a new offset.
64 *
65 * @param newPosition
66 * position to move the channel to. The next read will start from
67 * here. This should be a multiple of the {@link #blockSize()}.
68 * @throws java.io.IOException
69 * the position cannot be updated. This may be because the
70 * channel only supports block aligned IO and the current
71 * position is not block aligned.
72 */
73 public void position(long newPosition) throws IOException;
74
75 /**
76 * Get the total size of the channel.
77 * <p>
78 * Prior to reading from a channel the size might not yet be known.
79 * Implementors may return -1 until after the first read method call. Once a
80 * read has been completed, the underlying file size should be available.
81 *
82 * @return r total size of the channel; -1 if not yet available.
83 * @throws java.io.IOException
84 * the size cannot be determined.
85 */
86 public long size() throws IOException;
87
88 /**
89 * Get the recommended alignment for reads.
90 * <p>
91 * Starting a read at multiples of the blockSize is more efficient than
92 * starting a read at any other position. If 0 or -1 the channel does not
93 * have any specific block size recommendation.
94 * <p>
95 * Channels should not recommend large block sizes. Sizes up to 1-4 MiB may
96 * be reasonable, but sizes above that may be horribly inefficient. The
97 * {@link org.eclipse.jgit.internal.storage.dfs.DfsBlockCache} favors the
98 * alignment suggested by the channel rather than the configured size under
99 * the assumption that reads are very expensive and the channel knows what
100 * size is best to access it with.
101 *
102 * @return recommended alignment size for randomly positioned reads. Does
103 * not need to be a power of 2.
104 */
105 public int blockSize();
106
107 /**
108 * Recommend the channel maintain a read-ahead buffer.
109 * <p>
110 * A read-ahead buffer of approximately {@code bufferSize} in bytes may be
111 * allocated and used by the channel to smooth out latency for read.
112 * <p>
113 * Callers can continue to read in smaller than {@code bufferSize} chunks.
114 * With read-ahead buffering enabled read latency may fluctuate in a pattern
115 * of one slower read followed by {@code (bufferSize / readSize) - 1} fast
116 * reads satisfied by the read-ahead buffer. When summed up overall time to
117 * read the same contiguous range should be lower than if read-ahead was not
118 * enabled, as the implementation can combine reads to increase throughput.
119 * <p>
120 * To avoid unnecessary IO callers should only enable read-ahead if the
121 * majority of the channel will be accessed in order.
122 * <p>
123 * Implementations may chose to read-ahead using asynchronous APIs or
124 * background threads, or may simply aggregate reads using a buffer.
125 * <p>
126 * This read ahead stays in effect until the channel is closed or the buffer
127 * size is set to 0.
128 *
129 * @param bufferSize
130 * requested size of the read ahead buffer, in bytes.
131 * @throws java.io.IOException
132 * if the read ahead cannot be adjusted.
133 */
134 public void setReadAheadBytes(int bufferSize) throws IOException;
135 }